This invention relates to the integration of ball and fluid film bearings to avoid high ball bearing loads and to obtain long bearing life in high speed turbomachinery, and more particularly to the unloading of the lateral loads from the ball bearings in high speed turbomachinery such as the high pressure oxygen turbopump of the space shuttle main engine to extend the bearing life and the speed range of such turbomachinery.
The high pressure oxygen turbopump used in the space shuttle main engine uses duplex ball bearings, i.e., a pair of bearings mounted closely adjacent each other, near each end of the rotor. A damping seal such as that disclosed in U.S. Pat. No. 4,545,586, adjacent the pump end of the rotor besides reducing leakage between the fixed and rotating portions also damps rotor lateral motion and thus shares the bearing load in the pump portion of the turbopump. However, the ball bearings in the turbine portion of the turbopump have no load sharing provision. The duplex bearings are each a pair of ball bearings having an axial preload force applied thereto to avoid ball skidding and to produce high bearing stiffness. The preload is provided by a spring located between the outer races of each pair of duplex ball bearings. The high bearing stiffness prevents seal rubbing and instability due to whirl, that is, the orbiting of the rotor shaft due to fluid forces acting to urge the shaft tangentially, when radially from the rotational center, creating an eccentric rotation.
Due to ball bearing wear the operational time of the high pressure oxygen turbopump of the space shuttle main engine is limited, thereby hindering the space shuttle program. The excessive ball bearing wear is a result of excessive lateral or side loads. Such side loads frequently may not be shared equally between the bearing units of one duplex bearing pair because of shaft bending, deformation of the bearing holders and other factors. The forces on the rapidly rotating impeller may cause the side loads on a single bearing to be in excess of twice the bearing axial preload, which is substantially greater than the generally recommended one-half or one-third loading. Thus, the balls are forced up and down the shoulders of the bearing races cyclically as the ball train rotates at approximately 43 percent of the shaft speed. These excursions of the balls cause ball skidding, excessive heating and excessive wear of the bearings. Additionally, during turbopump start-up and shut-down axial overloads are present which require that the balls have large contact angles to counteract these axial forces. On the other hand, the large lateral or side loads during operation require small contact angles if over-stress is to be prevented. Where the outer races are permitted to float and the balls have large contact angles, spinning of the outer races occurs and load shearing by the bearing with the damping seal is limited. Accordingly, compromises in the bearing design have been made which have resulted in the aforesaid limitations in bearing wear.